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<title>Debugging with GDB: Writing an Xmethod</title>

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<a name="Writing-an-Xmethod"></a>
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<hr>
<a name="Writing-an-Xmethod-1"></a>
<h4 class="subsubsection">23.2.2.15 Writing an Xmethod</h4>
<a name="index-writing-xmethods-in-Python"></a>

<p>Implementing xmethods in Python will require implementing xmethod
matchers and xmethod workers (see <a href="Xmethods-In-Python.html#Xmethods-In-Python">Xmethods In Python</a>).  Consider
the following C<tt>++</tt> class:
</p>
<div class="smallexample">
<pre class="smallexample">class MyClass
{
public:
  MyClass (int a) : a_(a) { }

  int geta (void) { return a_; }
  int operator+ (int b);

private:
  int a_;
};

int
MyClass::operator+ (int b)
{
  return a_ + b;
}
</pre></div>

<p>Let us define two xmethods for the class <code>MyClass</code>, one
replacing the method <code>geta</code>, and another adding an overloaded
flavor of <code>operator+</code> which takes a <code>MyClass</code> argument (the
C<tt>++</tt> code above already has an overloaded <code>operator+</code>
which takes an <code>int</code> argument).  The xmethod matcher can be
defined as follows:
</p>
<div class="smallexample">
<pre class="smallexample">class MyClass_geta(gdb.xmethod.XMethod):
    def __init__(self):
        gdb.xmethod.XMethod.__init__(self, 'geta')
 
    def get_worker(self, method_name):
        if method_name == 'geta':
            return MyClassWorker_geta()
 
 
class MyClass_sum(gdb.xmethod.XMethod):
    def __init__(self):
        gdb.xmethod.XMethod.__init__(self, 'sum')
 
    def get_worker(self, method_name):
        if method_name == 'operator+':
            return MyClassWorker_plus()
 
 
class MyClassMatcher(gdb.xmethod.XMethodMatcher):
    def __init__(self):
        gdb.xmethod.XMethodMatcher.__init__(self, 'MyClassMatcher')
        # List of methods 'managed' by this matcher
        self.methods = [MyClass_geta(), MyClass_sum()]
 
    def match(self, class_type, method_name):
        if class_type.tag != 'MyClass':
            return None
        workers = []
        for method in self.methods:
            if method.enabled:
                worker = method.get_worker(method_name)
                if worker:
                    workers.append(worker)
 
        return workers
</pre></div>

<p>Notice that the <code>match</code> method of <code>MyClassMatcher</code> returns
a worker object of type <code>MyClassWorker_geta</code> for the <code>geta</code>
method, and a worker object of type <code>MyClassWorker_plus</code> for the
<code>operator+</code> method.  This is done indirectly via helper classes
derived from <code>gdb.xmethod.XMethod</code>.  One does not need to use the
<code>methods</code> attribute in a matcher as it is optional.  However, if a
matcher manages more than one xmethod, it is a good practice to list the
xmethods in the <code>methods</code> attribute of the matcher.  This will then
facilitate enabling and disabling individual xmethods via the
<code>enable/disable</code> commands.  Notice also that a worker object is
returned only if the corresponding entry in the <code>methods</code> attribute
of the matcher is enabled.
</p>
<p>The implementation of the worker classes returned by the matcher setup
above is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">class MyClassWorker_geta(gdb.xmethod.XMethodWorker):
    def get_arg_types(self):
        return None

    def get_result_type(self, obj):
        return gdb.lookup_type('int')
 
    def __call__(self, obj):
        return obj['a_']
 
 
class MyClassWorker_plus(gdb.xmethod.XMethodWorker):
    def get_arg_types(self):
        return gdb.lookup_type('MyClass')

    def get_result_type(self, obj):
        return gdb.lookup_type('int')
 
    def __call__(self, obj, other):
        return obj['a_'] + other['a_']
</pre></div>

<p>For <small>GDB</small> to actually lookup a xmethod, it has to be
registered with it.  The matcher defined above is registered with
<small>GDB</small> globally as follows:
</p>
<div class="smallexample">
<pre class="smallexample">gdb.xmethod.register_xmethod_matcher(None, MyClassMatcher())
</pre></div>

<p>If an object <code>obj</code> of type <code>MyClass</code> is initialized in C<tt>++</tt>
code as follows:
</p>
<div class="smallexample">
<pre class="smallexample">MyClass obj(5);
</pre></div>

<p>then, after loading the Python script defining the xmethod matchers
and workers into <code>GDBN</code>, invoking the method <code>geta</code> or using
the operator <code>+</code> on <code>obj</code> will invoke the xmethods
defined above:
</p>
<div class="smallexample">
<pre class="smallexample">(gdb) p obj.geta()
$1 = 5

(gdb) p obj + obj
$2 = 10
</pre></div>

<p>Consider another example with a C++ template class:
</p>
<div class="smallexample">
<pre class="smallexample">template &lt;class T&gt;
class MyTemplate
{
public:
  MyTemplate () : dsize_(10), data_ (new T [10]) { }
  ~MyTemplate () { delete [] data_; }
 
  int footprint (void)
  {
    return sizeof (T) * dsize_ + sizeof (MyTemplate&lt;T&gt;);
  }
 
private:
  int dsize_;
  T *data_;
};
</pre></div>

<p>Let us implement an xmethod for the above class which serves as a
replacement for the <code>footprint</code> method.  The full code listing
of the xmethod workers and xmethod matchers is as follows:
</p>
<div class="smallexample">
<pre class="smallexample">class MyTemplateWorker_footprint(gdb.xmethod.XMethodWorker):
    def __init__(self, class_type):
        self.class_type = class_type

    def get_arg_types(self):
        return None

    def get_result_type(self):
        return gdb.lookup_type('int')

    def __call__(self, obj):
        return (self.class_type.sizeof +
                obj['dsize_'] *
                self.class_type.template_argument(0).sizeof)
 
 
class MyTemplateMatcher_footprint(gdb.xmethod.XMethodMatcher):
    def __init__(self):
        gdb.xmethod.XMethodMatcher.__init__(self, 'MyTemplateMatcher')
 
    def match(self, class_type, method_name):
        if (re.match('MyTemplate&lt;[ \t\n]*[_a-zA-Z][ _a-zA-Z0-9]*&gt;',
                     class_type.tag) and
            method_name == 'footprint'):
            return MyTemplateWorker_footprint(class_type)
</pre></div>

<p>Notice that, in this example, we have not used the <code>methods</code>
attribute of the matcher as the matcher manages only one xmethod.  The
user can enable/disable this xmethod by enabling/disabling the matcher
itself.
</p>
<hr>
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